# TDO2

Source: https://onco.cc/targets/tdo2/  
OnCo record `tdo2` (Target). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

TDO2 does the same job as IDO1, breaking down tryptophan into kynurenine, which quietens T cells. It is one reason IDO1 inhibitors alone may not have been enough; no TDO2 drug has reached late trials.

## Summary

TDO2 (chromosome 4q32.1) is a haem-dependent dioxygenase that catalyses the oxidative cleavage of the L-tryptophan pyrrole ring, converting it to N-formyl-L-kynurenine (UniProt P48775). IDO1 and TDO2 catalyse the commitment step of the kynurenine pathway; kynurenine activates the aryl hydrocarbon receptor, generating immune-tolerant dendritic cells and regulatory T cells and a tumour microenvironment defective at recognising cancer cells (Cheong and Sun 2018, Trends Pharmacol Sci). No TDO2-selective agent was found in ClinicalTrials.gov under the names the corpus knows.

## Fields

- Kind: Target
- Last checked: 2026-09-24
- Also known as: tryptophan 2,3-dioxygenase; TDO
- Tags: checkpoint-map
- Symbol: TDO2
- Class: enzyme
- Biology: The liver-type tryptophan dioxygenase; UniProt records no tissue distribution for the human protein, so none is claimed here.
- Where found: Not recorded by UniProt for the human protein

## Notes

- Prevalence not recorded: HGNC and UniProt carry no positivity rates.

## Sources

- HGNC HGNC:11708: https://www.genenames.org/data/gene-symbol-report/#!/hgnc_id/HGNC:11708
- UniProt P48775: https://www.uniprot.org/uniprotkb/P48775/entry
- NCBI Gene 6999: https://www.ncbi.nlm.nih.gov/gene/6999
- Cheong and Sun 2018 (Europe PMC): https://europepmc.org/article/MED/29254698

## Connected records

- targets: [IDO1](https://onco.cc/targets/ido1/)
- drugs: [Epacadostat](https://onco.cc/drugs/epacadostat/)
- pathways: [The cancer-immunity cycle](https://onco.cc/pathways/cancer-immunity-cycle/)
- terms: [Checkpoint (two meanings)](https://onco.cc/terms/checkpoint/)

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JSON: https://onco.cc/api/v1/entities/tdo2.json